This invention relates generally to methods for modulating light and more specifically to methods and arrangements for producing modulated light having linear gray scale in light modulating systems with a plurality of states, wherein the response time for the light modulator to modulate between the states may be longer than the duration of at least one of the time periods used to produce a desired gray scale intensity.
It is well known that humans viewing successive images within short time intervals may perceive the images as a single or continuous image. For instance, cinematic motion pictures include a series of individual images; however, the individual images appear as a continuous image when viewed in succession above a certain frame frequency. This frequency has been called the critical flicker frequency and in many systems, the critical flicker frequency is roughly 60 hertz. Thus, in most situations, when the time interval for each image in a series is on the order of {fraction (1/60)}th of a second, the individual images become indiscernible.
Certain display systems exploit this concept to produce images. For example, consider a display system consisting of an array of pixels, each pixel having only two states, ON and OFF. This type of display system is know as a binary display system. In such a system, the pixels switch between the two states, thus modulating light so as to produce images. Binary display systems are used in a variety of applications, including head-mounted, hand-held, desk-top and projection devices. Consider further that this display system is capable of switching the individual pixels between the two states at frequencies much greater than the critical flicker frequency. If a specific pixel is ON for half of the time and OFF for half of the time and the frequency of modulation is less than the critical flicker frequency, the pixel appears to flash. However, if the pixel modulates between ON and OFF at a frequency greater than the critical flicker frequency, then the pixel appears to be ON continuously, but the intensity appears to be half as great as the intensity if the pixel was in the ON state. Likewise, a pixel that is ON for one-fourth of the time and OFF for three-fourths of the time appears to have one-fourth the intensity of the pixel being always in the ON state, assuming the frequency of modulation is greater than the critical flicker frequency.
This intensity variation in light modulating systems such as the one described above is known as gray scale. The greater the number of different intensities the system is able to produce, the greater the level of gray scale the system is said to have. In order to maximize the number of different intensity levels a system produces, the framexe2x80x94the time period during which a single image is producedxe2x80x94is typically divided into time segments or slots. In one common example, the duration of each slot is determined such that each slot is twice as long as the next shortest slot, and the total duration of all slots combined is equal to the frame duration. Each slot is then assigned to be either ON or OFF. Thus, if the frame is divided into eight slots of unequal duration as explained above, (e.g., having duration ratios of 1:2:4:8:16:32:64:128), the slots may be assigned ON or OFF in 256 ways (28=256) to produce 256 unique intensities. Such a system is called an eight-bit gray scale system since the eight slots may be represented by eight binary bits with, for example, a 1 representing the ON state and a 0 representing the OFF state.
The demand to produce systems with more intensities, or greater levels of gray scale, is increasing as display system applications become more pervasive. However, if the system is incapable of modulating between states instantaneously, the speed with which the system switches between states may limit the level of gray scale the system is able to produce. For instance, if the response timexe2x80x94the time the light modulator takes to changes statesxe2x80x94is longer than the shortest slot, then the light may not be displayed for the correct amount of time during that slot to produce the desired intensity.
Display systems are not the only systems that encounter the gray scale limitation caused by the light modulating speed. Any multi-state light modulating system that has a non-zero response time to switch between states may experience this restriction. For example, referring initially to FIG. 1, one example of a basic system for modulating light and generally designated by reference numeral 10 is illustrated. Light modulating system 10 includes a light source 12, a light polarizer 14 and a light modulator 16. Light source 12 is configured to direct light 18 toward polarizer 14. Polarizer 14 is configured to pass light of one polarization state, for instance horizontally polarized light (i.e., horizontal with respect to the orientation of the polarizer). Horizontally polarized light H is then directed toward light modulator 16. For this example, light modulator 16 may be any binary light modulating system that has a non-zero response time to switch between states. In the present example, light modulator 16 has an ON state, wherein horizontally polarized light 20 is allowed to pass through to a viewing area 22, and an OFF state, wherein no light passes through to viewing area 22. The state of light modulator 16 is controlled by a drive signal from controller 24. Thus, light modulating system 10 is configured to produce a temporal pattern of light directed toward viewing area 22.
Having generally described the configuration and operation of light modulating system 10, a more detailed method for operating the system will now be described, continuing to refer to FIG. 1. As previously stated, light modulating system 10 is configured to produce a temporal pattern of modulated light directed toward viewing area 22. Depending upon the frequency with which the light is modulated, the pattern may appear to a human viewer as a series of flashes. This would occur, for instance, if the frequency of modulator 16 is less that the critical flicker frequency of the human eye. However, if the frequency is greater than the critical flicker frequency, then modulated light 20 would appear continuous and have an intensity corresponding to the fraction of time that modulator 16 is in the ON state. Thus, light modulating system 10 has the ability to vary the intensity of light 20 directed toward viewing area 22, even though the intensity of light source 12 remains constant.
Light modulating systems such as system 10 and methods for operating them are well known in the art. For example, light modulating system 10 may be a miniature display system of the type disclosed in U.S. Pat. No. 5,596,451, which is incorporated herein by reference. Further, U.S. Pat. No. 5,748,164, which is incorporated herein by reference, discloses several methods for using such a system to produce images having gray scale and/or color. However, as described above, if any slots are deficientxe2x80x94have duration shorter than the response time of the light modulatorxe2x80x94the system may not produce the desired intensity when the specific intensity level requires the light to be ON during that slot. Thus, the system may not produce a linear gray scale response. A linear gray scale response occurs when the ratio of any two input signals is equal to the ratio of the output intensities resulting from the two input signals.
For example, consider a four-bit gray scale system, including bits A, B, C, and D, each bit corresponding to a slot. Bit A, the least significant bit (LSB), determines the state (ON or OFF) of the shortest slot and has a time weight of 1; bit D, the most significant bit (MSB), determines the state of the longest slot and has a time weight of 8. The system is capable of providing 16 different intensities (24=16). Assuming a frame time period of {fraction (1/60)}th of a second, or 16.7 milliseconds, the duration of the slots associated with each bit are as follows: Bit Axcx9c1.1 milliseconds; Bit Bxcx9c2.2 milliseconds; Bit Cxcx9c4.4 milliseconds; and Bit Dxcx9c8.8 milliseconds. If the light modulator has a response time greater than 1.1 milliseconds, then the system will not properly display all 16 gray scale intensities. The reason for this is explained below.
Referring to FIGS. 2a-d, the drive signal and light modulator response for each of the four slots is illustrated for a system that has a response time greater than the LSB. FIG. 2a illustrates drive signal 30 and light modulator response 32 for bit D. In this example, drive signal 30 is in the OFF state prior to bit D, and bit D requires the light modulator to be in the ON state. Therefore, at the start 33 of the bit D slot, drive signal 30 transitions from the OFF state to the ON state. The transition in drive signal 30 causes the light modulator, as indicated by light modulator response 32, to begin transitioning from the OFF state to the ON state. The light modulator is not yet completely switched into the ON state for a period of time equal to the response time, indicated by reference numeral 34. In this example, drive signal 30 is in the OFF state after bit D. Therefore, at the end 35 of the bit D slot, drive signal 30 switches from the ON state to the OFF state, causing the light modulator to begin transitioning from the ON state to the OFF state as indicated by light modulator response 32. The light modulator is not yet fully switched into the OFF state until a period of time equal to response time 34 has passed.
Although it may appear that response time 34 would limit the light modulator""s ability to produce the desired optical response, this is not the case. The light modulator""s optical response as a result of bit D includes the entire period influenced by bit D drive signal 30, not just the light modulator response during the bit D slot. In other words, the optical response as a result of bit D is the integral of light modulator response 32 over the entire period influenced by bit D drive signal 30. This response equals the desired optical response that corresponds to the gray scale intensity represented by bit D being ON. FIGS. 2b and 2c provide similar illustrations for bits C and B, respectively.
FIG. 2d illustrates drive signal 36 for bit A and corresponding light modulator response 38. As indicated by drive signal 36, the desired light modulator state is OFF both before and after the bit A slot. At the beginning 40 of the bit A slot, the drive signal switches to the ON state, at which time the light modulator begins to transition to the ON state, as indicated by light modulator response 38. However, because the light modulator has a response time 34 greater than the duration of the bit A slot, the light modulator is not able to switch completely to the ON state before the end 42 of the bit A slot. Thus, at the end 42 of the bit A slot, the drive signal switches to the OFF state and causes the light modulator to begin transitioning back to the OFF state. In this case, however, the light modulator does not produce the desired optical response, as explained next.
In the three previous cases, the ON delay in the light modulator""s response at the end of the slot compensated for the OFF delay at the beginning of the slot. In the present case, the delays essentially overlap in time and the light modulator never reaches the fully ON state. Therefore, even though the delay at the end of the bit A slot partially compensates for the delay at the beginning of the slot, the two segments together are not of sufficient duration to produce the desired optical response. That is, the integral of light modulator response 38 over the period influenced by bit A drive signal 36 is less than the desired optical response that corresponds to the gray scale intensity represented by bit A being ON. Thus, conventional methods of producing gray scale such as this are limited in their ability to correctly produce linear binary gray scale in cases where the LSB slot time is shorter than the light modulator response time.
Referring now to FIGS. 3a and b, another factor is illustrated that further complicates efforts to produce linear gray scale in a binary system where, for illustration, the LSB slot time is shorter than the light modulator response time. FIGS. 3a and b illustrate timing diagram 50, drive signal 52 and light modulator response 54 for a case where the LSB, bit A, is positioned in time between bits D and C. In FIG. 3a bits D and C have a value of 0, representing the OFF state, while bit A has a value of 1, representing the ON state. As described above with reference to FIG. 2d, the light modulator, as indicated by light modulator response 54, is unable to completely transition to the ON state within the bit A slot time. Thus, the integral of the light modulator""s response over the period influenced by the bit A drive signal does not produce the desired optical response that corresponds to the gray scale intensity represented by bit A being in the ON state. The integral of the light modulator""s optical response in this case is represented by the region designated by reference letter X.
In FIG. 3b bit D has a value of 0, while bits C and A have a value of 1, as indicated by drive signal 56. When drive signal 56 reaches the point in time 57 when it represents bit C, the light modulator is still responding to the bit A signal. However, because bits A and C have the same value, the light modulator continues to transition toward the ON state. The integral of the light modulator""s response over the period influenced by the bit A drive signal is represented by reference letter Y. Although the LSB, bit A, has the same state in each of FIGS. 3a and 3b, the integrals of the light modulator""s response in each case, X and Y, are not equal. Thus, the light modulator""s response to the drive signal for bit A depends on the state of the light modulator before and after bit A. This factor further complicates the ability of conventional methods of producing linear gray scale in binary systems where the LSB slot time is shorter than the light modulator response time.
The present invention overcomes the aforementioned limitations and provides a method of producing light having linear gray scale in multi-state systems where at least one slot is shorter than the light modulator response time.
As will be described in more detail hereinafter, methods and arrangements for producing modulated light having grayscale are herein disclosed. The method includes providing a light modulator having grayscale based on a series of time intervals and having a plurality of light modulator states. The method also includes establishing the duration of each time interval such that the time intervals in the series have progressively varying duration. The method further includes determining a drive signal for each time interval that causes the light modulator to assume a specific light modulator state. The method further includes causing the light modulator to produce a desired time-averaged light level over the series of time intervals by in part driving the light modulator using the drive signal that corresponds to a particular time interval for a duration that is longer than the duration of the particular time interval, the particular time interval having duration shorter than the response time of the light modulator.
The method may also or alternatively include sensing the temperature of the light modulator and determining the duration by which the drive signal corresponding to the particular time interval exceeds the duration of the particular time interval based in part on the sensed temperature.
The method may also or alternatively include arranging the series of time intervals such that the light modulator is in the same state immediately prior to the particular time interval as the light modulator is in immediately after the particular time interval.
The method may also or alternatively include arranging the time intervals such that the particular time interval immediately follows a first part of a longer one of the time intervals and immediately precedes a second part of the longer time interval.
The method may also or alternatively include reducing the duration of the drive signal corresponding to the longer time interval by an amount of time that is related to the amount of time by which the drive signal corresponding to the particular time interval exceeds the duration of the particular time interval.
In one embodiment of the invention, a light modulator has grayscale based on a series of time intervals. The light modulator also has a plurality of light modulator states. The time intervals have progressively varying duration and each time interval has an associated drive signal that causes the light modulator to assume a specific light modulator state. The light modulator includes a controller that causes the light modulator to produce a desired time-averaged light level over the series of time intervals by in part driving the light modulator using the drive signal that corresponds to a particular time interval for a duration that is longer than the duration of the particular time interval, the particular time interval having duration shorter than the response time of the light modulator.
The light modulator also or alternatively includes a first arrangement that senses the temperature of the light modulator and a second arrangement responsive to the first arrangement that determines the duration by which the drive signal corresponding to the particular time interval exceeds the duration of the particular time interval based in part on the sensed temperature.
The light modulator also or alternatively includes a controller that arranges the series of time intervals such that the light modulator is in the same state immediately prior to the particular time interval as the light modulator is in immediately after the particular time interval.
The light modulator also or alternatively includes a controller for arranging the time intervals such that the particular time interval immediately follows a first part of a longer one of the time intervals and immediately precedes a second part of the longer time interval.
The light modulator also or alternatively includes a controller for reducing the duration of the drive signal corresponding to the longer time interval by an amount of time that is related to the amount of time by which the drive signal corresponding to the particular time interval exceeds the duration of the particular time interval.
The light modulator may be a ferroelectric liquid crystal display. Alternatively, the light modulator may be a nematic liquid crystal display, a plasma display or a micro-mechanical deformable mirror device.